Recently, in addition to ground waves, CATV systems and satellite broadcasting waves have been developed for the transmission of television signals. Using these new transmission techniques, the need for a system that can transmit stable high quality pictures has increased.
FIG. 4 is an example of a prior art high frequency receiving apparatus. As shown in the figure, a high frequency television signal entering from an input terminal 44 of a tuner unit is amplified in a high frequency amplifier 45, and is fed into one input terminal 46a of a mixer 46. A local oscillation frequency from an oscillator 49 enters the other input terminal 46b of the mixer 46. The differential frequency of the frequencies at these two input terminals is delivered from the mixer 46 as an intermediate frequency signal to an intermediate frequency filter 50. The frequency of the oscillator 49 is controlled by a phase locked loop (PLL) circuit 48 based on the natural frequency of a reference solid-state oscillator element (usually a quartz oscillator) 47. The intermediate frequency output of this tuner unit is input to a picture detector, described below, through the intermediate frequency filter 50.
In the picture detector, an input intermediate frequency signal from the intermediate frequency filter 50 is amplified in an intermediate frequency amplifier 51. The output of the intermediate frequency amplifier 51 is fed into one input terminal 52a of a video synchronous detector 52 and one input terminal 56a of a phase comparator 56. The output of the phase comparator 56 passes through a low pass filter 57 and goes into a detection oscillator 54. The output of the detection oscillator 54 is applied to a first phase shifter 55 for shifting its phase by +.pi./4(rad) and a second phase shifter 58 for shifting its phase by -.pi./4(rad). The output of the first phase shifter 55 is applied to the other input terminal 56b of the phase comparator 56, and the output of the second phase shifter 58 is applied to the other input terminal 52b of the video synchronous detector 52. The output of the video synchronous detector 52 is fed to an output terminal 53.
During picture detection, the intermediate frequency signal which is the output of the tuner unit is amplified in the intermediate frequency amplifier 51, and its output is divided into two inputs, one of which is applied to one input terminal 56a of the phase comparator 56, and is shifted in phase by +.pi./4(rad) in the first phase shifter 55 and is compared with the output of the detection oscillator 54 received from the other input terminal 56b. The output of the detection oscillator 54 is deviated in phase by -.lambda./4(rad) from the intermediate frequency signal, and is combined with a phase deviation of -.lambda./4(rad) in the second phase shifter 58. A signal deviated by -.pi./4(rad) from the intermediate frequency is added to the other input terminal 52b of the phase synchronous detector 52, and the intermediate frequency of the output of the intermediate frequency amplifier 51 is switched thereby causing synchronous picture detection.
In this type of conventional construction, if there is a steep change in waveform in the direction of deepening the degree of amplitude in amplitude modulation, i.e., if the video signal changes steeply to a white level as shown in part A in FIG. 5(a), to reproduce the original waveform correctly when this waveform is demodulated, it is important that the phase stability be high even if the amplitude of the video carrier entering the phase comparator 56 decreases suddenly. In the conventional detection oscillator 54, in order to cope with the offset of the video carrier frequency, in the detection characteristic, it is of utmost importance to protect the drawing stability of the picture detector against fluctuations of intermediate frequency. Thus, a detection oscillator having low phase stability is used. In the prior art, the oscillation frequency variable range of the detection oscillator 54 is set at about .+-.200 kHz, a tuning circuit having a low Q is used, and the phase stability is low. As a result, the output of the picture synchronous detector 52 has an overshoot in the white picture signal region due to nonlinearity of input, an output as shown in part B in FIG. 5(b), and linear picture detection is poor.
The prior art described above does not provide the important advantage of superior linear picture detection even when there is a sudden change in the amplitude of the video carrier frequency.